Installation 01 was a Halo ring previously Monitored by 007 Greenie Spark. It was lost when he set the ring on a random course vector slipspace jump, and because of this the order of the Halo rings needed to be re-arranged.
Another ring was given the designation Installation 01, and Installation 12, the ring owned by Guilty Guilty, Greenie's nemesis, was renamed Installation 04. The new Installation 01 was mostly oceanic with just a few islands to house the major structures on board.
The original Installation 01 remains lost, and was never recovered for the entire time the League is in power in the Galaxy, so we can assume it will never be recovered. The only remnent of Installation 01 that remains is the duplicate of Greenie Spark that remained in the Forerunner Domain to engineer their downfall and rescue Cortana.